1. Field of the Invention
The present invention relates to a method of recording a servo signal and more particularly to a novel and highly accurate method of recording a servo signal which is suitable for use with a hard disk or the like. It also relates to apparatus for carrying out the method.
2. Description of the Prior Art
A method of tracking a hard disk by using an external sensor is disclosed in Japanese laid-open patent publication No. SHO 63-154916. In this method, a Voice coil motor is driven so as to move a head arm. With a position detector having a 4-phase optical sensor, the position of the head arm is obtained as a detection output. The head is tracked by comparing the detection output of each phase with a data track.
With only the external sensor, the accuracy of the tracking of the head is limited. To improve the accuracy, in a method that has been widely used thus far, the head is coarsely moved with the output of the external sensor and then it is finely moved to a target track with a servo signal which has been formed on the disk at the factory. Thus, if there is a deviation between the center of a track detected by the external sensor and the center of a data track on the disk because of deterioration due to age or some environmental change, it can in principle be compensated and the head can in principle be precisely tracked.
However, the servo signal mentioned above is written on the disk in accordance with the center of the track, which is detected by the external sensor after the drive unit has been assembled. Since the accuracy of the track pitches depends on the pitch accuracy of the external sensor, a problem of unevenness of pitches due to manufacturing tolerances arises. If the unevenness of pitches is pronounced, cross talk tends to occur, thereby degrading the read/write property. On the other hand, if the width of the guard band is increased to prevent the occurrence of cross talk, the track width is excessive and the disk storage capacity is not efficiently utilized.
To solve the problems mentioned above, the present applicant has proposed a method of writing a servo signal as disclosed in Japanese patent application No. SHO 63-308238. In this method, before a servo signal is written onto the disk, the unevenness of pitches intrinsic to the external sensor is checked. After the unevenness of pitches is compensated, the servo signal is written onto the disk. In other words, the unevenness of pitches intrinsic to the external sensor is compensated by moving the head from the center of the track detected by the external sensor a distance sufficient to effect the compensation and then writing the servo signal.
In the method of the earlier filed application, when the head is moved to effect the compensation while the output value of the external sensor is monitored, it is assumed that the output of the external sensor and the amount of movement of the head have a linear relationship. Actually, however, the relationship between the output of the external sensor and the amount of movement of the head is nonlinear. Thus, even after the head is moved for the purpose of effecting the compensation, there remains an unevenness of pitches to some degree.